What's with guys carrying machetes on Cap Metro bus?

I saw two guys get on the Cap Metro bus with backpacks with handles sticking out the top of the backpacks that looked like the handles of large knives (like a machete). They didn't look like the type that would be into play sword fighting in the park. Everything about them oozed with a feeling of anger and menace. They didn't say a word to each other the whole bus trip, they just kind of sat and glared and then got off together on Riverside.

I just want to know, is that common to see, and is there any legit reason people carry machetes in backpacks? Migrant farm works with tools? Homeless people for self defense?